Due to its ease of use the fiber Bragg grating (“FBG”) has been well accepted by the telecommunications and optical research communities for use in applications as a signal purifier and strain gauge. FBG-based devices, however, are sensitive to temperature and strain along the grating primary axis, which affects the resonance wavelength of the FBG. First, the optical fiber has a thermal expansion effect of its own. Second, the refractive index of the optical fiber also varies with temperature. In order to achieve more accurate measurement, the grating portions of such devices need to be athermal, but preferred materials for a FBG do not inherently have this characteristic. Fortunately, both the thermal expansion coefficient and the refractive index are linearly related to temperature, and by proper design thermal effects on an FBG can be minimized.
The resonance wavelength for an FBG follows the equation:λB=2neffΛ,  Eq. 1
where λB is the resonance wavelength, and Λ is the Bragg grating period. Thus, the variation to the resonance wavelength becomes:
                                          Eq            .                                                  ⁢            2                    ⁢                      :                    ⁢                                          ⁢                                    Δλ              B                        /            Δ                    ⁢                                          ⁢          T                =                ⁢                              2            ⁢                          (                              Δ                ⁢                                                                  ⁢                                                      n                    eff                                    /                  Δ                                ⁢                                                                  ⁢                T                            )                        ⁢            Λ                    +                      2            ⁢                                          n                eff                            ⁡                              (                                                      ΔΛ                    /                    Δ                                    ⁢                                                                          ⁢                  T                                )                                                                            =                ⁢                  2          ⁢                                    Λ              ⁡                              (                                                      (                                          Δ                      ⁢                                                                                          ⁢                                                                        n                          eff                                                /                        Δ                                            ⁢                                                                                          ⁢                      T                                        )                                    +                                      2                    ⁢                                                                                            n                          eff                                                ⁡                                                  (                                                                                    ΔΛ                              /                              Δ                                                        ⁢                                                                                                                  ⁢                            T                                                    )                                                                    /                      Λ                                                                      )                                      .                              
The term (Δneff/ΔT) is the temperature coefficient of refractive index and (ΔΛ/ΔT)/Λ) is the coefficient of thermal expansion of the optical fiber.
The thermal expansion coefficient for the optical fiber is approximately 0.55×10−6/° C. and the exact value can usually be found from manufacturers' data sheets. The term (Δneff/ΔT), however, is less often provided by manufacturers and therefore usually needs to be verified experimentally.
For example, the inventors have conducted experiments in which they have gotten a value of dneff/dT=9.8×10−6/° C. Per Eq. 2, this gives the total variation of:
                                                        Δλ              B                        /            Δ                    ⁢                                          ⁢          T                =                ⁢                  2          ⁢                      Λ            ⁡                          (                                                (                                      Δ                    ⁢                                                                                  ⁢                                                                  n                        eff                                            /                      Δ                                        ⁢                                                                                  ⁢                    T                                    )                                +                                                                            n                      eff                                        ⁡                                          (                                                                        ΔΛ                          /                          Δ                                                ⁢                                                                                                  ⁢                        T                                            )                                                        /                  Λ                                            )                                                              =                ⁢                  2          ×          0.5          ⁢                                          ⁢          μm          ×                      (                                          9.8                ×                10                            -                              6                /                °C                            +                              1.448                ×                0.55                ×                                                      10                                          -                      6                                                        /                  °C                                                      )                                                  =                ⁢                  10.6          ⁢                                          ⁢          pm          ⁢                      /                    ⁢          °C                    
where neff=1.448 and Λ=0.5 μm are used.
To make a device athermal means to make the term ΔλB/ΔT vanish. It is obvious, however, that this cannot be accomplished by using optical fiber alone. Therefore, a new approach is needed.